Rename gitter channel

[fedefernandez]

It looks renaming rooms are not fully supported. The way to do it is to create a new one, write an email to support to migrate the messages and then delete the old one.

https://gitlab.com/gitlab-org/gitter/webapp/issues/1162

As part of this issue, we should verify the conf to point to the right room name

mu-scala/project/ProjectPlugin.scala

Line 394 in 9743fca

{ info => GitterBadge.apply(info.copy(owner = "47deg", repo = "mu")) },